Notes from the morning-after press conference
Posted by Emily Lakdawalla
04-07-2005 10:55 CDT
Here in Von Karman auditorium at JPL, as they get ready for the press conference, they are playing "Rock Around the Clock," by Bill Haley and His Comets. Very appropriate!
The press panel is mostly familiar: Andy Danztler, Rick Grammier, Shyam Bhaskaran, Mike A'Hearn, and Pete Schultz.
Danztler: "Do I have a cool job or what? The Discovery program's goals are really to enable ground-breaking science, to push the envelope of what is thought to be possible. We do that to inspire the public, and to inspire future generations of engineers. It's been a good day for us all."
Grammier: "I'd like to report that the team is very tired. However, they are very excited and feeling very proud at this moment." A billion hits on their website!? "In terms of status, I'm happy to report to you that as of this morning we're still sure we don't have an impactor spacecraft. On the other side, the flyby spacecraft is still in great shape." Shows a gorgeous lookback image of the plume and debris from crater formation being backlit by the Sun. It's awesome. "The flyby is in good shape, all subsystems are green. Every iota of memory storage is totally full. We are working as fast as we can to get all the data downlinked and sent to the science team. All in all, it's still performing like a champ."
Bhaskaran: shows a graphic of the targeting of the impactor. Pre-release was up and to the left of the comet -- only barely missing. "The first ITM occurred and the post ITM 1 was actually further away. This wasn't entirely unexpected, yet it does make us nervous, we don't like to see things like this, we were watching it closely. By ITM 2 it had pretty much gone exactly to the right place. The third ITM 3 started from the center of brightness but then we wanted to bias it to improve the view from the flyby spacecraft. It took us to a location close to where the flyby spacecraft flew. In the end, it turned out pretty good."
A'Hearn: "I'm going to show you first a new result that Jessica Sunshine got last night. We now have a map of the temperature of the nucleus immediately before the impact. We don't have the absolute calibration of the temperature yet, that'll take us another week or two. This'll be important to try to understand how rapidly the nucleus responds to changes in sunlight. More importantly, we're going to go back after the impact and find the cold spot where the crater is. But that'll take a lot of work.
"We're still doing lookback imaging, trying to see how long it takes before the impact event stops. It's clear that the ejecta were still coming out for at least several hours after the impact event. If there's a lot of volatiles there, the outgassing could go on for a long time. It could go on for weeks.
"First image is overexposed to look at coma. Surprising number of jets; fortunately they were faint.
Another image of nucleus, spectacular. "You look at that nucleus, and obviously it does not look like a pickle or a cucumber. It looks closer to a muffin. There is a lot of topographic relief. There are things on this comet that look a lot like impact craters to many of us. It looks very different from Wild-2 or Borelly. We don't understand what this means. This comet has had an orbital history that looks pretty much similar to Borelly and yet it looks totally different. There are lots of bright spots on that image. Most of them are just steep slopes facing the sun. A couple do not seem to be that, we don't understand them. There are some really smooth areas there, that curve around the side into darkness, and we don't understand that. Almost certainly there is layering, and that has to cause differences in topography. There are craters. And there are jets coming from somewhere which we hope to trace out.
Zoomed in view. "Again, you see lots of little bright areas. Nearly all of them are sun-facing slopes. Where the impactor went in is just above the lower of the two large craters in this image.
"The final image shows a little bit of detail at 4 meters per pixel. It's the last image on which you can identify features that were on the previous image. We've learned a lot from these images but we've learned even more from the cratering event itself."
Schultz: "Our cratering experiment went very very well. First image. It shows several important things. It shows an incandescent ball. Some is light being reflected from the Sun. If we didn't even have any light shining we would have taken our own flash picture. You see a long gash. That's not a gash, that's a shadow, of the plume passing across the surface.
"This was apparently, we may have been able to detect some structuring in the response to the impact. We've only had a couple of waking hours -- we were living on adrenaline.
"Even though we can't see the impact point behind the flash, we can see its shadow, and that points to the impact. My guess is there was soft layering on top, went down, finally got in contact with the ices, and went down there."
Time for press questions.
Any signs of the crater itself? A'Hearn: "We've certainly seen a feature where the crater would be. We haven't done enough image processing yet to be sure whether it's the crater itself or the shadow laid across the crater. We think we'll find the crater."
How different is the comet's inside from the shell? A'Hearn: "we certainly see big changes in the spectrum as the ejecta evolve from the initial stages. There are big differences in the composition."
Ice vs rock or other components? Schultz: "We can't answer that yet. We do see some changes, which is great, because we'll be able to resolve those differences. It'll take some debate on the science team. There was a lot of dust that came out. But that ratio, we need to do more spectroscopy, imaging, and observatory. It's going to take a long time to integrate everything. We had a lot of eyes watching."
In the lookback image, how far out was the cloud of ejecta? And what is the stuff made of? "27,000 kilometers away when that image was taken. Those ejecta go out thousands of kilometers at least. Whether they go further requires more image processing to know. It is largely dust. We see mostly dust in those images because that's what's reflecting the sunlight; by then the early flash has turned off. There is gas with it, but that's not actually what you see in the image."
Schultz: "This was probably a dusty soft surface on the very top; and then the stuff below...I don't think we saw a very thick hard crust. But we were looking from a distance, and the stuff that was thrown out was obscuring our view."
Angle of the ejecta? Schultz: "The central portion is very high angle. But there is a plume portion that is more inclined. We need to process the images, maybe get some stereo, see how the shadow is cast. We know we have an inclined curtain that was expanding out later on."
How close were you to being canceled? Danztler: "We do have to watch the money. Missions are competitively selected. They are selected on the basis of how much science per dollar they can do within a cost cap. If the mission is getting too close to the cost cap, it's a question of how far along is the project, what is the hope for recovery, is it still worth the money for the science? Some of the cost overrun was inflicted on the mission by HQ as a cost of the Genesis mishap. With Genesis we went to make sure that everything that was about to launch or already out in space, including Stardust, could benefit from everything we learned with the Genesis failure. That in itself cost a few million extra dollars. But when you're well on your way into a project, the science still looks like it's useful, we made the decision to go forward, and we are very glad we did."
A'Hearn: "I think it's going to be at least a week of careful image processing before we understand if the feature we have observed really is the crater itself. The problem is that there is this bright dust in front of it and we are looking through that, and you need to subtract that out of the picture. We may find it more easily by using the IR spectrometer and looking for temperature differences. And that's a very complicated procedure."
Betting pool? A'Hearn: "Everybody's pool can't be paid of for another week." Schultz: "I think we know whose money we're taking." A'Hearn: "Yes, we know some losers for sure."
Observatory data: A'Hearn: "Changes in abundances of gases, for sure. Changes in temperature."
Schultz: "At the moment of impact, you hear materials to extremely high temperatures. Some of that is heated vapor. Some of that is melt droplets from within the crater itelf. It is like a flashbulb -- material that is glowing so brightly that it illuminates its own picture."
Grammier: "It's not really paralleling the comet, it's contiunuing its orbit which is basically an elliptical orbit. I believe it's somewhere around 160 kg, that may be a little high, of useable fuel that is left. At this point we don't have a specific plan for an extended mission or a different mission. Once we get all the data down and finish the lookback imaging we'll mothball the spacecraft, put it into a sun-safe mode, where it'll still be positive in energy, but only the critical sequences will be running."
Schultz: "The experiments used regular ordinary garden perlite. The idea behind this was to have soemthing very high in porosity, very crushable, very low density. We noticed it would penetrate deeply, and then stuff would go back out; it goes deep and then it explodes because it's been stopped very quickly. There were other simulations that involved more gassy material. Now we have to go back to the drawing board."
"I think it's big. I don't think it's house sized, I think it's bigger than that."
"Prior to this, I thought we pretty well covered everything. Now that we've seen the impact, it's going to be a lot more fun to go back and see if we can match some of the constraints like the light curve, the timing. The NASA Ames Vertical Gun Range has been incredibly important. We've done experiments like this. We've seen this before. It happens when the projectile goes into the surface first and releases its energy below the opening. That's just the first guess. The initial flash is an umbrella-like vapor plume. And then we see the large column. And then the curtain. The curtain later on tells is that the crater is expanding slowly, like you would expect from loose material."
Similarities and differences to other comets? A'Hearn (wearily): "We knew before we went in that the composition you could measure from Earth is actually rather similar to Wild 2 and Borelly. Borelly had a bend in it. If you look at images of Tempel 1, it doesn't have a bend in it, one end looks round like a cylinder, the other looks concave. We don't have a shape model yet but we know we have a different shape. For the first time we have seen what we think are impact craters on the comet. There are some things on WIld 2 that some people think are impact craters, but many other people think they are not impact craters. We see these curving flat surfaces, which we do not understand. There are lots of differences, from the data we have so far."
A final announcement: in honor of the success, the original members of Bill Haley and His Comets will be performing out in front of JPL's administration building tomorrow! I'll have to come back for that.